1,2,3,5-Tetra-0-acetyl-4-thio-D-ribofuranose, prepared from 2,3,5-tri-O-benzyl-D-ribofuranose in four steps, was converted to the corresponding 2-chloroadenine nucleoside (8), which was deoxygenated to obtain 2-chloro-2'-deoxy-4'-thioadenosine (12). This is the first report of a 2'-deoxy-4,-thioribonucleoside of a purine rather than a pyrimidine. These novel nucleosides (8 and 12) were cytotoxic to several human tumor cell lines in culture. © 1994, Taylor & Francis Group, LLC. All rights reserved.